Spectrometers use light emission or absorption or Raman scattering by matter to qualify and quantify specific atoms and molecules in analysis of gas, solid or liquid phase compounds. In one case, the radiation emitted from a light source is absorbed with a particular energy determined by optical transitions occurring within the atoms, ions or molecules of an analyte. In another case, the light emitted by atoms, ions or molecules of the analyte is composed of spectral components of particular energy, which are determined by optical transitions within the atoms or molecules. In yet another case, light scattered by matter contains spectral components which are created by Raman scattering, corresponding to certain particular transitions in molecules or ions. For example, in infrared absorption spectroscopy, discrete energy quanta are absorbed by molecules due to excitation of vibrational or rotational transitions of the intra-molecular bonds.
Variations in environmental conditions as well as aging or fouling of reflector surfaces in a spectrometer sample cell, or replacement of fouled or deteriorated reflector surfaces can cause a beam path of a light source within a spectrometer to change over time or as a result of changing a reflector. Changes of the beam path in an optical spectrometer can invalidate the spectrometer calibration. In most cases, such spectrometers require factory calibration of at least a sample cell or replacement by a skilled technician. Such service calls and factory repairs are costly and result in downtime for the spectrometer and the operation it controls, while such repairs are being performed. This is a common problem today with conventional TDL (tunable diode laser) spectrometers which require a factory calibration of the sample cell when at least one reflector in the cell has to be replaced due to fouling or due to other deterioration of a reflecting surface. The factory turn-around time of such a sample cell repair and replacement has been precluding TDL spectrometers being used in many petrochemical production processes, such as ethylene and propylene production, due to unavoidable reactor upset conditions, which result in liquids flowing through sample cells and leaving damaging residue on reflectors.